Optical wavelength-division multiplexing (WDM) has now become the standard technique for increasing the transmission capacity of fiber-optic communications systems. Its development began in earnest with the introduction of the erbium-doped fiber amplifier (EDFA)—first reported in 1987 by Desurvire, Simpson and Becker, “High-gain erbium-doped fibre amplifier”, Optics Letters 12(11):888, and by Mears, Reekie, Jauncey and Payne, “Low-noise erbium-doped fibre amplifier operating at 1.54 μm”, Electronics Letters 23(19):1026—which could efficiently compensate for the losses generated by the multiplexing and demultiplexing processes. In systems employing wavelength division multiplexing, multiple channels located at different wavelengths are combined, with each channel being separately modulated to carry one or more information-bearing (e.g., voice, video or data) signals. In this manner WDM technology enables the total number of signals that can be simultaneously transmitted over a single optical fiber to be greatly increased.
Modern telecommunication systems rely heavily upon transmission of optical WDM signals across long spans of optical fiber. As the light propagates from a transmitter to a receiver through the optical network, it loses energy in the transmission fiber itself (primarily due to light scattering) and also in the other fiber-optic components in the network (by more general loss mechanisms). In order to prevent this energy depletion, optical signals are repeatedly amplified at various points in the network. The erbium-doped fiber amplifier is generally considered to be an optimal amplifier for optical signals in the wavelength range from about 1525 nm to 1570 nm. This range (sometimes referred to as the C-band) is the most frequently used band for optical communications as discussed, for example, in volumes (IIIA and IIIB) of ‘Optical Fiber Telecommunications”, edited by I. P. Kaminow and T. L. Koch, Academic Press (1997). Indeed, EDFA's are regularly used in almost every part of a fiber-optic network: as power boosters in transmitters, as in-line amplifiers in long fiber spans, as pre-amplifiers in receivers, and the like.
An erbium-doped waveguide amplifier, or EDWA (a recent example of which has been described in U.S. Pat. No. 6,157,765 by A. J. Bruce and J.Shmulovich), has properties similar to those of an EDFA. Unlike EDFAS, however, EDWA's are waveguides that are manufactured on planar substrates using glass hosts that may differ dramatically in composition from those used in EDFAS. Although less efficient, in many instances EDWA's have advantages compared to EDFAs. For example, a packaged EDWA chip has a much smaller size than a packaged EDFA. Moreover, it is natural and straightforward to integrate an EDWA with other passive or active optical components on the same planar substrate, which is impossible with EDFAS. Also, in some instances the integrated module may be able to perform functions that are not achievable by its fiber-optic analog.
In designing an EDFA or an EDWA a number of basic principles must be taken into account, some of which will be presented below to better facilitate an understanding of the present invention. In a typical EDFA, the gain per unit length at a given position x along its length and at a wavelength λ is given by the following expressionγ(x,λ)=N1(x)γ*(λ)−N0(x)α(λ)≈α(λ){(N1(x)(η(λ)+1)−1},  (1)where N1 and N0 are the fractions of inverted and non-inverted Er ions, respectively (ideally, N1+N0=1), γ* is the gain coefficient of a fully inverted amplifier, α is the absorption coefficient of a non-inverted amplifier, and η is the ratio between γ* and α. N1 represents the inversion level of the amplifier and depends on both the pump and the signal power. Usually, at the input end of an EDFA the pump power is high and the signal power is low, therefore N1 is high and may reach 99%. However, at the output end the pump power is relatively low, whereas the signal power is relatively high, which results in a lower N1, which sometimes decreases to as low as 40%. In order to obtain the total gain G(λ) of the amplifier γ(x) must integrated over the length Λ of the amplifier:                                           G            ⁡                          (              λ              )                                =                                                    ∫                0                Λ                            ⁢                                                γ                  ⁡                                      (                                          x                      ,                      λ                                        )                                                  ⁢                                  ⅆ                  x                                                      =                                          〈                                  γ                  ⁡                                      (                    λ                    )                                                  〉                            ⁢              Λ                                      ,                            (        2        )            where the average gain per unit length (γ(λ)) is given by                               〈                      γ            ⁡                          (              λ              )                                〉                =                              1            Λ                    ⁢                                    ∫              0              Λ                        ⁢                                          α                ⁡                                  (                  λ                  )                                            ⁢                              {                                                                                                    N                        1                                            ⁡                                              (                        x                        )                                                              ⁢                                          (                                                                        η                          ⁡                                                      (                            λ                            )                                                                          +                        1                                            }                                        ⁢                                          ⅆ                      x                                                        =                                                            α                      ⁡                                              (                        λ                        )                                                              ⁢                                                                  {                                                                                                            〈                                                              N                                1                                                            〉                                                        ⁢                                                          (                                                                                                η                                  ⁡                                                                      (                                    λ                                    )                                                                                                  +                                1                                                            )                                                                                -                          1                                                }                                            .                                                                                                                              (        3        )            
It is apparent from Eq.3 that the spectral shape of the gain is completely determined by the average inversion level of the amplifier. This equation also holds for an amplifier consisting of two or more stages, provided that these stages all employ doped fibers having the same composition. If some amplifier stages have different doping levels, Eq.3 needs to be modified as shown below. Changes in the Er doping level, Er distribution, or the fiber cross-section affect primarily the amplitude of the absorption spectrum (α(λ) ), which in turn modifies only the magnitude of the gain and leaves its spectral shape unaffected. However, any change in the nature of the glass fiber that hosts the Er ions does affect the shape of both the α(λ) and η(λ) spectra, and thus may also alter the shape of the gain spectrum. It should be noted that these conclusions are applicable to any EDFA and are therefore very important in the design of multistage in-line amplifiers.
FIG. 1 shows a typical multistage in-line EDFA (only three stages of which are shown), consisting of separate gain sections or stages 101, 102 and 103 that have respective gains Gk, k=1,2,3 . . . n (where n is the total number of stages in the amplifier). Lossy optical elements 104, 105, 106 and 107 may be positioned between the amplifier stages, which have losses L0, L1, L2 . . . , Ln, respectively. All the components are connected with single mode fibers 108. L0 and Ln are losses at the input and output of the EDFA, respectively, which include losses due to isolators, taps, pump couplers and so on. L1, L2 . . . , Ln−1 include losses due to optical components such as filters, variable optical attenuators (VOA), dispersion compensation modules (DCM), add-drops and other components that are sometimes located between amplifier stages. As a result, the total gain (in dB) of the EDFA is given by                               G          tot                =                              -                          L              0                                +                                    ∑                              k                =                1                            n                        ⁢                                          (                                                      G                    k                                    -                                      L                    k                                                  )                            .                                                          (        4        )            
Each stage can also be characterized by its respective noise figure f1 through fn, which is defined as the ratio between the input and output signal-to-noise ratios (SNR), or by its counterpart F1 through Fn, which is expressed in units of dB. Various noise sources contribute to the degradation of the signal-to-noise ratio; in an EDFA a primary source is the so-called signal-spontaneous beat noise, which determines the noise floor of the amplifier. The output noise figure, which can be calculated from a knowledge of the noise figures of each individual stage, can be expressed in the form (see, for example, “Erbium-Doped Fiber Amplifiers”, by E. Desurvire, Wiley and Sons Inc., New York, 1994):                               f          tot                =                              ∑                          k              =              1                        n                    ⁢                                    f              k                                                      ∏                                  i                  =                  0                                                  k                  -                  1                                            ⁢                                                           ⁢                                                l                  i                                ·                                  g                  i                                                                                        (        5        )            where li and gi are the i-th stage loss and gain (g0=1) measured in linear units (for comparison Li and Gi are in dB). Since ligi>>1 for i>0, it is clear from Eq. 5 that the first stage of the amplifier has the strongest effect on the overall noise figure and that the last stage has the least effect. Therefore, an in-line amplifier having multiple stages allows the different stages to be optimized for different functions. For example, a two-stage EDFA may consist of a preamplifier optimized to have a low noise figure and a power amplifier optimized to have a high output power.
Under typical operating conditions, Gtot is equal to the span loss, which is generally about 20-26 dB, so that the output signal power for 40 WDM channels is about 20 dBm. This implies that for the three-stage amplifier in FIG. 1, the 3rd stage operates in deep saturation and its gain is highly compressed, resulting in a typical noise figure in excess of 6 dB. On the other hand, the first two stages are typically highly inverted and are characterized by a low noise figure of about 4 dB.
As previously mentioned, for a variety of reasons it is often desirable to place various passive fiber-optic components between the amplifier stages. For example, a DCM may be inserted between the 1st and the 2nd stages and a gain-flattening filter between the 2nd and the 3rd stages. While other optional passive elements also may be present between the stages, one element that is almost always required between the stages of an in-line EDFA is a gain-flattening filter to optimize the amplifier performance to provide a broadband, nearly wavelength independent, signal gain, i.e. a spectrally flat gain. The best gain flatness is often achieved by inserting a gain-flattening filter between the amplifier stages, i.e., by introducing a wavelength-dependent loss Lk(λ):                                                         G              tot                        ⁡                          (              λ              )                                =                                                    〈                                  γ                  ⁡                                      (                    λ                    )                                                  〉                            ⁢                              Λ                tot                                      -                                          ∑                                  k                  =                  0                                n                            ⁢                                                L                  k                                ⁡                                  (                  λ                  )                                                                    ,                            (        6        )            where Λtot is the total length of Er fiber in the EDFA and (γ(λ)) is the average gain per unit length as determined by (N1) in Eq. 3. It is not necessary to have a gain-flattening filter between every stage, one filter is often sufficient for the whole amplifier.
The gain-flattening filter is usually optimized for operating conditions corresponding to some fixed average inversion level (N1), which is governed by fixed values of Gk and Lk. Thus, if the amplifier is reconfigured (e.g., by inserting or removing a passive component between amplifier stages) so that the value of the loss Lk is changed, this in turn alters the average inversion level (N1), and the resulting gain spectrum is generally no longer flat. As a result an active element such as a variable optical attenuator (VOA) is generally required to actively adjust the interstage losses so that the gain flatness of the EDFA can be controlled. Such an active element can preserve the gain flatness of the amplifier by counter-compensating for the change in operating conditions. For example, if a DCM having an insertion loss of 10 dB is replaced with a different DCM having an insertion loss of 5 dB, the VOA can be tuned in front of the DCM to provide an additional 5 dB loss, so that the overall loss (and hence the gain) is not altered. While the VOA can maintain the gain flatness of the EDFA while providing flexibility in the amplifier configuration, the additional cost and complexity of a VOA, as well as the cost associated with the active control of the VOA, becomes problematic.
It would therefore be desirable to provide a method and apparatus by which optically lossy elements can be inserted, removed or interchanged between one or more stages of a multi-stage optical amplifier without the need for a variable optical attenuator.